<script language="javascript" type="text/javascript">
    function validarBusca() {
        var usuario = document.buscar.nome.value;
        if (usuario == "") {
            alert('Preencha o campo antes de pesquisar!');
            return false;
        }
    }
</script>
<form name="buscar" action="../controller/buscarpessoa.php" method="GET" onsubmit="return validarBusca()">
    <div id="info">
    <input type="text" name="nome" size="20">
    <input type="submit" name="buscar" value="Buscar Headbangers">
    </div>
</form>